The present invention relates generally to the field of digital image processing, and more particularly to a technique for enhancing the appearance of digital images, such as medical diagnostic images, to more clearly render certain image features while maintaining overall image intensity levels. The invention also provided a system and method for enhancing medical diagnostic images, particularly computed radiology and digital x-ray images.
Digital imaging systems have become increasingly useful in a variety of fields, particularly in medical diagnostic imaging. In systems of this type, image data is generated that defines characteristics of discrete picture elements or pixels of an image matrix. The matrix of pixels, when viewed by the observer, forms an overall reconstructed image in which features of interest are visible to the observer. In general, the characteristics of the pixels that form the image may be intensity (i.e. gray level), color and the like. In medical diagnostic imaging, for example, discrete pixel images are acquired, processed, and displayed typically by varying the intensity of the pixels making up the image matrix to simulate conventional photographic images, or to render images which would have been difficult or impossible to obtain through conventional means.
Digital imaging systems have been configured to process image data in various manners, depending upon the source of the data, the type of data, and the display technique to be employed. For example, in the medical diagnostics field, image data may be acquired through various modality systems, including magnetic resonance imaging (MRI) systems, computed tomography (CT) systems, x-ray systems, ultrasound systems, and so forth. Depending upon the imaging modality, the image data may be further processed, filtered, enhanced, scaled, and so forth to reduce noise and to render more visible particular features of interest. The resulting image may be viewed by a user, such as on a computer monitor or similar display, often referred to as softcopy, or may be output as hard copy, such as on a paper or similar support, or photographic film.
With the advent of softcopy displays, problems have presented themselves which did not occur, or were less pronounced with conventional technologies. For example, in x-ray images rendered on photographic film, a light box was typically used by radiologists to read the resulting images, with excellent contrast and detail being provided by the photographic film. Where softcopy is desired, however, the same features may be less visible due to the limited dynamic range of the display device, or to the inherent limitations in rendering the detail in a manner that enables features of interest to be distinguished comparably to photographic film. This problem is particularly acute in view of the fact that softcopy displays are increasingly popular due to the archiving of large volumes of images in institutions, and to the facility of accessing the images for display at workstations where the radiologist will page through a large number of examinations during a reading session.
In the case of x-ray images obtained from computed radiography (CR) or digital detectors, a high dynamic range is often present in the image due to the inhomogeneity in the attenuation of various materials in the path of radiation used to generate the image data. The image dynamic range is typically larger than the dynamic range of a softcopy display device. This mismatch in dynamic ranges is further exacerbated by the inherent contrast lost due to multiple wavelengths of the x-ray source. Moreover, some x-ray photons will traverse the subject directly, while other photons will be diverted from their path by intervening material. This phenomenon, often referred to as scatter, can result in additional loss of contrast, particularly when the images are viewed on softcopy displays.
Methods have been proposed for controlling or adjusting dynamic range and contrast enhancement, particularly in CR images. These include multi-resolution algorithms, adaptive histogram equalization methods, multi-channel filtering techniques, local range modification techniques, contrast-limited adaptive histogram equalization techniques, xe2x80x9cjust noticeable differencexe2x80x9d adaptive contrast enhancement techniques, and so forth. While adequate for certain applications and situations, these techniques have not, however, provided a satisfactory overall process for generalized image enhancement, but tend to be extremely specific in nature to a body part or image type.
There is a need, therefore, for an improved image enhancement technique which can be applied to digital image data to improve contrast and feature discernability in images, particularly in the medical diagnostics field. There is a particular need, at present, for a technique which may be applied in a straightforward manner to image data for reconstructed images to be viewed on softcopy displays by compressing a dynamic range, while improving the contrast-to-noise ratio without enhancing image artifacts.
The present invention provides an image enhancement technique designed to respond to these needs. The invention may be applied in a variety of environments, but is particularly well suited to medical diagnostic imaging, wherein image data is displayed on a softcopy displays such as computer or workstation monitors. The technique offers the advantage of being applicable both to new image viewing systems or stations, as well as to existing systems by loading of image enhancement software for carrying out the functions envisaged.
In a presently preferred embodiment, the technique first performs an adaptive equalization operation, in which pixel values are equalized from original values, while maintaining overall appearance of lighter and darker regions corresponding to higher and lower intensity values. Adaptive local contrast enhancement is then performed based upon the equalized values to render detailed structures more discernable. The contrast enhancement preferably generates a mid-range boosted image, then performs non-linear mapping of the mid-range boosted image onto the dynamic range of the softcopy display device. Several different candidate mapping functions may be available for this purpose, the appropriate function being selected based upon the dynamic range of the display device.